Flak is a type of projectile mainly used in anti-air warfare. Flak burst in mid air causing area-effect damage and hitting several enemies at once. It is slow and does little damage per shot, but the fact that it can hit several targets at once makes up for this. Due to these characteristics, flak is very powerful against Gunships and transports, as well as Tech 1 and Tech 2 units, due to their low health and speed.
Flak is great against transports as it can destroy its cargo if not the whole transport easily. Likewise, against concentrated gunships (especially in the campaigns) flak is necessary to quickly dispatch the closely packed masses of units.
However, Flak has a hard time hitting tech 3 air superiority fighters as they are too fast and manueverable, and SAMs are best suited for killing them. However a good mixture of air defences will give the best result, and it is not a good idea to swap out all of the T2 flak defences for SAMs – while very powerful, T3 anti air will struggle to thin out large air fleets, T3 gunships (Broadsword, Wailer, Restorer) are fairly prevalent.
If you are cybran, you can use your transport to send a deceiver and engineers to build some flak near an undefended air factory to destroy it's creations as your engineer captures it.
Flak is mounted for every faction on all T2 anti-air flak artillery defences and T2 mobile AA flak artillery from the Land factory. Factionally, the Seraphim T2 Cruiser and Battleship (as the Olaris AA Artillery Cannon), the Ythotha, and the Cybran Megalith are also armed with it. It is notable that as well as the Seraphim Navy, they and the Aeon also have hover flak; Cybran commanders without experimentals as well as UEF commanders will have to make do with static turrets.